GS1508-CTA Ver la hoja de datos (PDF) - Gennum -> Semtech
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GS1508-CTA Datasheet PDF : 8 Pages
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GS1508
1.000
SDO
0.800
0.600
SDO
0.400
0.200
Fig. 5 Static Protection Diodes
OUTPUT RETURN LOSS MEASUREMENT
To perform a practical return loss measurement, it is
necessary to force the GS1508 output to a DC high or low
condition. The actual measured return loss will be based on
the outputs being static at VCC or VCC-1.6V. Under normal
operating conditions the outputs of the GS1508 swing
between VCC-0.4V and VCC-1.2V, so the measured value of
return loss will not represent the actual operating return
loss.
A simple method of calculating the values of actual
operating return loss is to interpolate the two return loss
measurements. In this way, the values of return loss are
estimated at VCC-0.4V and VCC-1.2V based on the
measurements at VCC and VCC-1.6V.
The two values of return loss (high and low) will typically
differ by several decibels. If the measured return loss is RH
for logic high and RL for logic low, then the two values can
be interpolated as follows:
RIH = RH- (RH-RL)/4 and
RIL = RL+(RH-RL)/4
where RIH is the interpolated logic high value and RIL is the
interpolated logic low value.
For example, if RH = -18dB and RL = -14dB, then the
interpolated values are RIH = -17dB and RIL = -15dB.
OUTPUT AMPLITUDE ADJUSTMENT
The output amplitude of the GS1508 can be adjusted by
changing the value of the RSET resistor as shown in Figure 6
and Table 1. For an 800mVp-p output with a nominal ±7%
tolerance, a value of 53.6% is required. A ±1% SMT resistor
should be used.
The RSET resistor is part of the high speed output circuit of
the GS1508. The resistor should be placed as close as
possible to the RSET pin. In addition, PCB capacitance
should be minimized at this node by removing the PCB
groundplane beneath the RSET resistor and the RSET pin.
0.000
0.0000
0.0050
0.0100
0.0150
1 / RSET (mho)
0.0200 0.0250
Fig. 6 GS1508 Output Amplitude vs. 1/RSET
TABLE 1: RSET vs VOUT
RSET R
500.0
150.0
75.0
53.6
43.2
1/RSET
0.0020
0.0067
0.0133
0.0187
0.0231
OUTPUT SWING
0.095
0.298
0.575
0.785
0.937
NOTE: For reliable operation of the GS1508 over the full
temperature range, do not use an RSET value below 49.9Ω.
5
522 - 02 - 03
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